This invention relates generally to the field of servo circuits for controlling a motor moving an apparatus. The apparatus includes means for picking up signals indicative of the actual position of the apparatus which are used in deriving the control signals fed to the motor, thus completing a servo loop. The invention will be described with respect to a magnetic disk drive for the storage of digital data in which a servo loop is used to control the motion of the read/write head with respect to the disk surface, and wherein the read/write head picks up permanently encoded position information written on the magnetic disk. Such servo controls of disk drives are well known and typically involve analog filtering techniques to provide stability to the servo loop; that is, by filtering out resonant frequencies and the like. Such analog filtering techniques involve the difficulty that the filter means may be subject to component drift over time or with temperature change, and may therefore not be adequate to fully control the loop. Furthermore, analog filtering may not be compatible with the remainder of the signal processing used to derive the position control signal which may, in today's technology, comprise microprocessor means for generating the command signal. Accordingly, it would be desirable to provide improved filter and command signal processing means for use in a servo loop. In particular, one which could readily be interfaced with the means used to generate the control signals.